The present invention relates to a method for operating an electrical machine in a short-circuit operation mode, in particular for application in a motor vehicle.
Furthermore, the present invention relates to a device for operating an electrical machine in a short-circuit operation mode, in particular for use in a motor vehicle.
Finally, the present invention relates to a motor-vehicle drive train having at least one electrical machine in a short-circuit operation mode and having a device for operating the electrical machine of the type mentioned above.
A method such as this and a device such as this are known from the laid-open specification DE 10 2006 003 254 A1.
It is generally known in the field of motor-vehicle drive-train technology to use an electrical machine as sole drive or together with a drive motor of another type (hybrid drive). Hybrid drives typically consist of a combination of an internal combustion engine and at least one electrical machine and the associated energy stores in the form of a fuel tank and a battery. Different types of hybrid drives exist, wherein a distinction is made between two basic structures, more particularly between series and parallel hybrid drives. A combination of both structures is also designated a power-split hybrid drive.
The series hybrid drive is characterized by a series circuit of energy converters. This requires for example two electrical machines and one internal combustion engine. One of the electrical machines is operated as a generator; the other electrical machine is operated as a motor. The internal combustion engine itself is not connected to a drive train of the motor vehicle. It charges up the battery via the electrical machine operated as a generator and/or directly provides the required electrical energy to the electrical machine operated as a motor. The power which is necessary for the locomotion of the motor vehicle is therefore finally transferred to the drive train from the electrical machine operated as a motor.
The parallel hybrid drive is characterized by both an internal combustion engine and an electrical machine being able to transmit the mechanical powers thereof to a drive train. By means of a mechanical coupling of the two machines and the drive train, the powers can be added together. The possibility of said power addition enables a relatively small dimensioning of the two machines, without disadvantages in terms of driving performance arising for the motor vehicle.
There are different ways of implementing parallel hybrid drives. One possibility is to connect the electrical machine directly to a crankshaft of the internal combustion engine (crankshaft starter generator) or to couple the electrical machine with the internal combustion engine by means of a belt drive. The two drive sources can therefore be used both together and individually for the locomotion of the motor vehicle. The electrical machine can also selectively be operated as a generator or as a motor.
The power-split hybrid drive is characterized by a combination of the principles which have already been illustrated. The use of a power-split transmission (planetary transmission) makes it possible to transmit a portion of the power of the internal combustion engine directly, that is to say mechanically, to the drive train while the remaining portion of the power is converted into electrical energy by means of a generator. Said electrical energy can in turn be stored in a battery or be transmitted directly to an electrical machine arranged downstream of the transmission. In the case of the power-split hybrid drive, both the electrical machine and the internal combustion engine can be used for the locomotion of the motor vehicle.
A power electronics unit typically serves to control electrical machines in a motor vehicle and in particular in a hybrid vehicle. Said system includes an inverter, which converts a DC voltage or a direct current from the (high-voltage) battery on board the motor vehicle into an alternating current. The power electronics unit conducts a high voltage of typically 60 volts.
If a fault occurs in the electrical machine, the machine is transferred into a safe state. The safe state for the electrical machine is typically the active short-circuit operation mode.
Document DE 10 2006 003 254 A1, mentioned at the outset, describes an electrical machine having a power electronics unit in a short-circuit operation mode. To handle a fault, the electrical machine is first of all switched into a freewheeling mode and then into a short-circuit operation mode.
In the case of the active short-circuit operation mode, motor windings of the electrical machine are shorted together by means of the power electronics unit. The short-circuit current which occurs owing to the short-circuit operation mode results in the electrical machine receiving a relatively small torque and therefore only slightly influences a drive train. However, if the electrical machine is erroneously put into a freewheeling mode instead of the active short-circuit operation mode, the torque (braking torque) received by the electrical machine increases. Said increased torque can, in particular at high rotational speeds, lead to the drive train being braked to the effect that a destabilization of the vehicle may result. In addition, a battery of the high-voltage system can be unintentionally charged as a result and damaged as a result.